Oceanography

3.3.1.Tidal bore

In some tidal rivers, which are shallow, steep and funnelshaped, the tide advances as a single roaring wall of water, called tidal bore. Here, the duration of rise of the tidal wave decreases very much and the rise occurs suddenly in the form of a breaking wave, which travels upstream with its steep, wall-like front. As the tide enters the river, it is slowed down by the constriction. Additional water entering the river continuously catches up with the initial water, with the result that a Churning wall of water moves up the river. The formation of a tidal bore depends upon the strength of the incoming tidal wave, slop and depth of the channel and river flow.

Tidal bores may move at speeds of up to 25km/h and have heights up to 8m. The most famous bore in North America is the bore of the Petticodiac river at the head of the Bay of Fundy. In this river, which empties into the Bay of Fundy at Moncton, New Brunswick, Canada, the tide comes in as a “bore” i.e., as an almost vertical wall of water, which may be as high as 1.5-2m. In India, on the east coast in the Hooghly river near Calcutta, the flow of the tide is occasionally marked by abrupt wave called the “tidal bore”. Hooghly is the major river in India which experiences tidal bores. In the Hooghly, when bore forms, the colour of the water where the breaks indicates a very concentrated syrup of sand water mix. On the west coast of India, in the upper parts of the Gulf of Cambay, tidal bores form near the entrances of Sabarmati river and Mahi river. One of the bores travels up the Sabarmati river, while another one travels up the Mahi river. Each of these bores sweeps through the respective channel as a line of disturbed and breaking water. The bore in Mahi river may be about 2.5m in height and attains a speed of 18km/h at the highest spring tides. The bore in Sabarmati river is similar but less pronounced.